1. Field of the Invention
The present invention relates to extracorporeal lithotripsy devices, and in particular to such devices which utilize a locating system, such as an x-ray or ultrasound system, top identify the position of the calculus within the patient and to control a positioning system for the shock wave tube so as to position the shock wave tube such that its focus is coincident with the calculus.
2. Description of the Prior Art
Lithotripsy devices are known which include a shock wave tube for generating shock waves focussed at a location disposed a distance in front of the shock wave tube. The shock wave tube includes a rigid portion and a deformable portion filled with coupling fluid, and means for positioning the shock wave tube at the exterior of a patient so that the deformable portion is pressed against the patient to conform to the patient's body, with the focus of the shock wave tube and the calculus being coincident. The shock wave tube and the patient support table may be moved relative to each other for this purpose by the positioning system. A locating system is also provided for identifying the position of the calculus within the patient, such as an x-ray system or an ultrasound system. A visual display is provided by the locating system which includes a mark identifying the calculus and an indicator for the position of the focus. Devices of this type are utilized, for example, for disintegrating kidney stones in situ in the human body, and have the advantage of avoiding invasion of the body using instruments.
Such an apparatus is described, for example, in German OS No. 33 28 051, corresponding to U.S. Pat. No. 4,674,505. This apparatus includes a shock wave tube having a tubular jacket, a flat coil, and a copper membrane separated from the flat coil by an insulating foil. An acoustic convergent lens is disposed in the tubular jacket which focusses the planar shock waves generated by the membrane to a focal point. For coupling the shock wave tube to the patient, the opening of the tubular jacket at the opposite end from the membrane is closed with a deformable cover. The volume between the cover and the membrane is filled with a coupling fluid. As the shock wave generator is moved toward the patient until the calculus to be disintegrated is situated at the focus of the lens arrangement, the cover is pressed against the surface of the patient and is deformed thereby, so that the space behind the cover within the shock wave tube is completely filled with coupling fluid, so that the shock waves will always propogate within the fluid.
For obese patients, however, it is possible that the calculus to be destroyed is so far inside the body of the patient that the focal distance of the lens arrangement is not adequate. As a result, during the automatic positioning of the shock wave tube with respect to the patient, the rigid portion of the shock wave tube, or the lens arrangement, may come into contact with the patient. Since coincidence of the focal point and the calculus has not been achieved, the positioning system will continue to try to move the shock wave tube to bring about such coincidence, with the result of pushing the patient and thus displacing the calculus from its original position, which was used as the basis for positioning the shock wave tube. Since the calculus has now been moved from this original position, all subsequent positioning of the shock wave tube is no longer accurate.